1. Field of the Invention
The present invention relates to a sealed-type secondary battery, and in particular to a material of the sealed-type secondary battery.
2. Description of the Related Art
In the related art, there is known a battery pack in which a plurality of battery modules including a plurality of unit cells are arranged and integrally constrained by endplates on both ends. A battery pack is a secondary battery (nickel-metal hydride battery or lithium ion battery) which is used as a power supply of an electric automobile and a hybrid automobile. In order to secure safety and simplify maintenance, the battery pack is set as a sealed type in which gas occurring inside the battery is not discharged to the outside of the battery system and which does not require replenishment of the liquid.
FIG. 7 is a perspective view of a unit cell 4 which is a part of a sealed-type secondary battery disclosed in JP Hei 9-120801 A. An electrode plate 11 in which a plurality of positive electrode plates and negative electrode plates are alternately arranged with a separator therebetween and an alkali electrolyte solution are stored in a rectangular battery container 1 made of a synthetic resin, and a lid 12 made of a synthetic resin is adhered to an upper-portion opening of the battery container 1. The lid 12 comprises a rib 14 on an outer surface of a wide side wall 13, similar to a rib 5 of the battery container 1, and the lid 12 and the battery container 1 are adhered to each other by heating and fusing. On the lid 12, a positive electrode terminal 16, a negative electrode terminal 17, and a safety valve 18 are fixed. The fixing of the safety valve 18 is achieved by adhesion through heating and fusing to the lid 12, similar to the battery container 1. A lead piece 19 of the negative electrode plate is connected through welding to a hanging portion at a lower end of the negative electrode terminal 17, and an upper portion of the negative electrode terminal 17 is fixed to the lid 12 in a liquid-tight and air-tight manner. The document discloses that, in such a structure, a polymer alloy having polyphenylene ether resin and a polyolefin resin as primary constituents is used as the synthetic resin of the container including the battery container 1 and the lid 12.
The polymer alloy is superior in mechanical strength such as bending modulus of elasticity compared to the general-purpose polyolefin resin, and thus the battery characteristics can be improved without being damaged by creep deformation. On the other hand, the polymer alloy is more expensive than the general-purpose polyolefin resin, and is disadvantageous for mass production in view of the cost. In addition, there is also a problem in the molding in that the molding component cost is also high, such as high percentage of foreign object deficiency due to generation of carbides.